Why do you need feedback from the camera? When you electrically press the shutter you know when it will take the picture exactly by trial and error, about 250ms later. If you are using the ML intervalometer, you can move the slider at the same time. Why would you want to use the USB host shield to control the camera if you are just pressing the button anyway? I have done this, and you can control more parameters. But why would you want to? I'm sure you have a reason, I'm just asking.